A conventional computed tomography (CT) system includes a radiation source, a detector array, an object positioning unit between the source and the detector, a computer unit with image processing subsystems, and a color graphic image display subsystem. The radiation source is either an x-ray tube, an x-ray linear accelerator, or a gamma-ray emitting radioisotope which emits a flux of photons. The photons from the radiation source are highly collimated (focused) to form a thin fan-shaped beam which is directed at the object of interest (to be examined). The fan beam is typically adjustable, for example from 10 to 35 degrees wide and from 1 to 5 mm thick.
During a scan of the object, high energy photons from the radiation source passing through the object are highly collimated upon entering the detector array. The detectors convert the photons into visible analog light events, and these are then digitized by proprietary current-integration electronics into datasets. The scanned datasets are computer processed to calculate density matrices, in order to electronically reconstruct the object's image in the plane of the beam. The reconstructed image is passed onward to graphics display routines for analysis and video display. The image information is typically analyzed with proprietary software programs to extract precise density and dimensional information.
Tomograms of defects are developed by rotating the object in the radiation beam, or the source-detector arrangement, to provide opacity measurements along many interior axes. A typical scan includes thousands of measurements. Projection data computed over 180.degree. (a series of scans along a plane starting at one side and continuing to the other--i.e. .+-.90.degree. from a middle) produce an image which is a two-dimensional cross section. Three-dimensional images are then generated by making successive scans along the height of the object.
Nuclear reactor vessels and the like have circular walls with relatively large thicknesses of steel. Due to this thickness, a typical long metal chord length of 48"-63" is encountered through which the photons of any practical source cannot pass to complete the desired scans. Thus, a conventional computed tomogram is not practical for such large and thick walled vessels. Obviously, the extreme size and weight of the vessel also makes it difficult to perform the necessary 180.degree. scans.
It will also be appreciated that conventional radiographs of such vessels currently used have problems with detecting of cracks of negligible width perpendicular to the beam direction. This is due to negligible density changes in the vessel wall under inspection.